[llvm] [SelectionDAG] Simplify vselect true, T, F -> T (PR #100992)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon approved this pull request.

LGTM - the riscv-gurus still need to confirm what to do regarding using verify-machineinstrs in rvv tests

https://github.com/llvm/llvm-project/pull/100992